The enzymes of cyclic nucleotide metabolism are different in retina from those in other tissues and are active early in embryogenesis. High cycle GMP levels are present in retinas of dogs with retinal degeneration indicating the possible involvement of cyclic GMP in the etiology of the degenerative process. BIBLIOGRAPHIC REFERENCES: Krishna, G., Krishnan, N., Fletcher, R., and Chader, G.: Effects of light on cyclic GMP metabolism in retinal photoreceptors. J. Neurochem. 27: 717-722, 1976. Chader, G., Fletcher, R., O'Brien, P., and Krishna, G.: Differential phosphorylation by GYP in isolated rod outer segments of the retina. Biochemistry 15: 1615-1620, 1976.